KATHMANDU, Nepal — A noted American ski mountaineer, Hilaree Nelson, is missing after apparently falling into a crevasse on Monday while attempting to ski down a peak in Nepal.

Also on Monday, an avalanche lower down the same mountain, Manaslu, killed at least one person and injured 14, some critically, on a separate expedition, according to Nepal’s tourism department.

On Monday morning, Ms. Nelson and her climbing and romantic partner, Jim Morrison, stood atop Manaslu, the world’s eighth-tallest peak.

“Summit success!” they shouted, crying, hugging and taking photos with their three Sherpa guides, who used a satellite phone to share the good news with Jiban Ghimire, owner of Shangri-La Nepal Trek, the outfitter that organized their expedition.

Then, at the 8,163-meter (26,781-foot) peak, the couple strapped on their skis and pointed them down the slope. Fifteen minutes later, the guides radioed Mr. Ghimire once again: “big problem.”

They said Ms. Nelson, 49, appeared to fall into a 2,000-foot crevasse. Whether Ms. Nelson, an athlete and mother of two based near Telluride, Colo., survived the fall was unknown. After the incident, Mr. Morrison skied safely to base camp seeking help. He arrived around four and a half hours after the celebratory pictures were taken from the mountain’s peak.

On its website, the North Face, her sponsor, said of Ms. Nelson, “With a career spanning two decades that includes dozens of first descents through more than 40 expeditions to 16 different countries, Hilaree Nelson is the most prolific ski mountaineer of her generation.”

The incidents highlighted the extreme risks taken in Nepal by mountaineers and the local guides who support them and comprise an outsize share of injuries and deaths on the peaks. The Sherpas, members of an ethnic group in Nepal known for their skill at high-altitude climbing, fix rope, carry supplies and establish camps.

They are often the barrier between the foreign teams who hire them and death on the mountain, frequently expending their health or even lives to protect visiting climbers.

Ms. Nelson’s career has included working as a guide herself on some of North America’s toughest peaks.

On Monday, weather conditions on the mountain quickly changed, as they often do on Nepal’s top peaks, and an avalanche hit a separate expedition of climbers lower down the mountain, killing one and injuring 14. Four people who were critically injured were evacuated from base camp by helicopter.

On foot, “it takes three days to reach the incident site from base camp,” said Mr. Ghimire. “Weather is hampering search and rescue operations.”

He said the company would send a helicopter at first light Tuesday morning to the crevasse where Ms. Nelson disappeared. Mr. Morrison, who reached base camp in good health, will be on the helicopter for the rescue mission if the weather is conducive to flying, Mr. Ghimire said.

Bigyan Koirala, a government tourism department official, said the chance of a rescue was slim.

“Based on the briefings and difficult terrain, it’s really hard to say whether we will be able to rescue her alive,” he said.

The couple’s first major ski mountaineering expedition was in 2017, when they traveled to the Indian Himalayas to attempt the first ski descent of 21,165-foot Papsura, known as the Peak of Evil.

They did it, completing “an icy, 3,000-foot, 60-degree virgin ski descent with almost no visibility,” according to the North Face.

Ms. Nelson was the first woman to climb two 8,000-meter peaks, Everest and Lhotse, in one 24-hour push.

She returned to make history in Nepal with Mr. Morrison in 2018, when they were the first to successfully ski down Lhotse, the world’s fourth-highest peak at 27,940 feet. In the wake of their achievement, they decided to ski down another 8,000-meter peak, Manaslu, according to Mr. Ghimire — a tried but still extremely technically challenging feat.

They were far from alone on the mountain this fall. Hundreds of aspiring climbers were drawn to Manaslu after Mingma Gyalje Sherpa rediscovered the mountain’s true summit, the place at the top of the mountain just above the fore summit, which mountain researchers said had not been reached in autumn since a Japanese expedition in 1976.

Among those with permits this year were members of a rarefied group of climbers who have successfully climbed the world’s 14 highest peaks, and who had counted Manaslu as a successful summit until Mingma G., as the Nepali climber is known, found the true peak.

Manaslu is among the world’s most treacherous mountains, and dozens of people have died over the hundreds of recorded attempts to reach the summit. In 2019, an avalanche on the mountain killed nine climbers.